The present invention relates to gas transportation, and more particularly, the present invention relates to a natural gas composition transport system and method.
Currently, there are three basic means to transport natural gas or natural gas liquids (NGL).
The first and usually the most practical and feasible means of transporting natural gas and NGL, is a pipeline. Geographic obstacles are generally the only reasons that a pipeline is either not practical or feasible.
The second most common mode of natural gas transportation is liquified natural gas (LNG). Unfortunately, LNG has a high capital cost associated and requires a cryogenic liquefaction plant proximate the source, and a re-gasification plant at the discharge point. A liquefaction plant is generally very large and therefore, not practical or feasible for offshore gas production.
The third mode of transporting natural gas and liquids is by compression of the natural gas (CNG) and compressed natural gas liquids (CNGL). Unfortunately, conventional CNG concepts use steel or steel lined pressure vessels. This usually creates a weight problem as pressure vessel wall thickness is proportional to the design pressure. Steel pressure vessels are usually very heavy, especially when the design pressure is high.
At high pressures, the rupture characteristics of steel pressure vessels becomes a safety concern. In the event of a pressure vessel failure, the likelihood exists for other surrounding vessels to fail. Thus, the nesting of steel or steel lined pressure vessels is not a favourable option. As a concomitant disadvantage, without nesting the storage bottles together, economy of space is not maximized.
Further problems encountered by the use of steel or steel lined pressure vessels are corrosion and certification. To ensure that a minimum wall thickness is maintained, steel pressure vessels are typically x-rayed by use of radiography. This testing requires sufficient space to set up the necessary equipment. This, inherently, eliminates nesting as a possibility.
Several CNG ship-based pilot projects have been carried out in the past. Unfortunately, it was found that the weight of the steel pressure vessels required to make a potential project feasible was too heavy for the ships. The excessive weight created stability problems and water draft concerns. Also, servicing such a heavy ship would be impossible in any of the world""s existing dry-docks. Exemplary of the prior art in this regard is U.S. Pat. No. 5,803,005, issued Sep. 8, 1998 to Stenning et al.
Currently, there are truck-mounted CNG projects in operation, but they are subsidized, since they would not be economically feasible on their own. The capacity of steel pressure vessels used for truck-mounted CNG systems is limited by a weight constraint of a truck transportation system. In addition, to store and transport raw gas and liquids, which most likely would be corrosive, requires a corrosion allowance. This further decreases the capacity, since the allowable operating pressure requires commensurate reduction.
There are communities that are currently tied-in to a natural gas pipeline which experience cyclic demands which the pipeline cannot handle. In such areas, there is a requirement to supply natural gas during peak load times. Unfortunately, current means of CNG transportation are not feasible due to highway weight restrictions, which in turn restricts the amount of natural gas that can be carried on a per trip basis.
Ethane would be an ideal hydrocarbon to use as an additive to meet the cyclic requirements of such areas. It can be transported as a liquid under relatively high pressure (700 psi) thus a significant quantity may be transported per trip. Under pipeline distribution pressure, ethane will exist as a vapor and therefore mix with the natural gas. As sales gas normally contains slightly less than 10% ethane, an incremental quantity will not adversely affect the consumers use of the gas. Nozzle tips would not require adjustment to account for the additional proportion of ethane. As a concomitant advantage, ethane has a higher heat value than the main constituent of natural gas, which is methane.
Generally speaking, there are communities that are not currently tied into a natural gas network because the demand that would exist there does not justify the cost of a pipeline to that community, especially during the transition phase, which may last a number of years. In these cases, liquefied ethane by pressurization may allow for the gradual growth of a distribution system to a point where the cost of a pipeline is justified. It may even be feasible to continue trucking ethane in larger quantities. Methane or a mixture of methane and ethane may also be used, if a feasible means of transporting methane can be obtained.
To obtain a practical and feasible means of transporting CNG and or CNGLs, a safe and relatively light-weight pressure vessel is required. This would elevate the excessive weight related problems that make current steel or steel lined pressure vessels inappropriate for CNG transportation over water or land. In addition, a pressure vessel that is resistive to corrosion would allow the transportation of raw natural gas and NGLs. This would enhance the feasibility of a gas production project, by eliminating the requirement to first process the gas and or liquids at the source.
In view of what has been proposed previously, there exists a need for an improved system for transporting natural gas and natural gas liquids discussed herein. The present invention satisfies this need.
One object of the present invention is to provide an improved system and method for transporting CNG and NGL.
The use of composite pressure vessels will overcome the safety, weight, and corrosion-related problems mentioned above.
Rupture characteristics of composite pressure vessels are much more favorable than that of steel or steel lined pressure vessels. Composite pressure vessels also have a broader temperature range without strength reduction than conventional steel or steel lined pressure vessels. Thus, the use of comparable composite pressure vessels increases the level of safety over conventional steel or steel lined pressure vessels.
Comparable composite pressure vessels are much lighter (as much as 70% less) than conventional pressure vessels that are made of steel or are steel lined. Thus, the use of composite pressure vessels will overcome the weight-related problems associated with using conventional steel or steel lined pressure vessels.
Composite pressure vessels with a non-metallic liner such as high density polyethylene (HDPE) will not corrode. Composite pressure vessels with a HDPE liner will therefore overcome the corrosion-related problems of storing raw natural gas and or raw NGLs in conventional steel or steel lined pressure vessels.
A further object of one embodiment of the present invention is to provide a light-weight, ship-based system for transporting raw, unprocessed, compressed natural gas and or compressed natural gas liquid, the system comprising:
a ship;
a plurality of composite material pressure vessels for containing raw, unprocessed, compressed natural gas and compressed natural gas liquid;
connection means for interconnecting the vessels, the vessels being integrally connected to interchange pressurized gas and liquid from the vessels to user selected other vessels, whereby weight distribution in the ship may be adjusted.
The construction of composite pressure vessels varies from companies that manufacture them. The preferred model is constructed by wrapping an adhesive impregnated carbon fiber in a helical path around a HDPE liner. The ends of the pressure vessel would have an integral stainless steel boss to weld to external piping. This particular type of composite pressure vessel is known.
The transportation of gas and liquids together in multiple pressure vessels on a ship incurs a problem of its own as ballast on a ship is of a critical concern. If gas and liquids are loaded together, there may be an unfavorable distribution of weight incurred by the uneven distribution of liquids. Thus a system to transfer the liquids from certain or all composite pressure vessels, to specified pressure vessels, while under pressure, is required.
This problem may be overcome by orienting the composite pressure vessels vertically, so that the liquids will sit at the bottom. By connecting respectively the tops of the of pressure vessels to a gas manifold, and similarly connecting the bottoms of the pressure vessels to a liquid manifold, a slight pressure differential between the gas and liquids manifolds will facilitate a liquid/gas transfer. Thus, a desired weight distribution of the cargo may be obtained, and a stable and safe voyage may be conducted.
Similarly, a low center of gravity would be desired for a truck-mounted gas/liquids transportation system. This may also be obtained by orienting the composite pressure vessels vertically and connecting the tops together as well as all the bottoms respectively. An equalization of the liquids would be similarly obtained and the lowest possible center of gravity will prevail.
The transportation of liquefied ethane may include conventional refrigeration and insulation to ensure that a liquid state is maintained.
The invention relates to a compressed natural gas (CNG) and compressed natural gas liquid (CNGL) transportation system designed to operate at ambient temperatures. The CNG/CNGL transportation system may be either ship-based, truck-mounted, or modular, whereby composite pressure vessels are used to overcome the safety limitations, weight restrictions and corrosion problems, related with steel or steel lined pressure vessels.
In the ship-based CNG/CNGL transportation system, composite pressure vessels are oriented vertically and aligned in banks. Each bank of cells is encased in a frame that is removable for maintenance if required. A module is made up of two or more banks of cells. Within each compartment of the ship, two or more modules may exist.
A further object of one embodiment of the present invention is to provide a method of transporting raw, unprocessed, compressed natural gas and compressed natural gas liquid aboard a ship, comprising the steps of:
providing a plurality of composite material pressure vessels for receiving gas or liquid within the ship;
providing connection means for interconnecting the vessels;
introducing the gas or liquid into the vessels; and
selecting vessels for receiving gas or liquid to adjust weight distribution of the vessels within the ship for transporting the gas or liquid.
All of the cells in a bank are connected to a gas manifold at the top and a liquids manifold at the bottom. A control valve exists at the end of each manifold. Each bank in a module is connected together by an upper gas manifold and a lower liquids manifold. Each manifold is connected to the main gas and liquids manifold of the ship by an isolation and control valve.
The ship-based CNG/CNGL transportation system incorporates a ballast control system by the integral connection of the banks and modules. Liquids that have gravitated to the bottom of the cells may be moved to specified cells using pressure differentials within the module or a liquids pump on the main liquids manifold. By this means desired ballast 10 may be obtained.
The loading of a ship-based CNG/CNGL transportation system is performed in two phases. Phase one is the loading of the gas and liquids directly into the composite pressure vessels using the source delivery pressure. Phase two is the loading of the gas and liquids at elevated pressures. On-board compression is used to compress the gas to this elevated pressure. During phase two, the oncoming liquids are removed with the assistance of deck mounted liquid slug catcher. The liquids are then pumped into designated cells to achieve the desired ballast.
Upon arriving at the unloading terminal, the gas and liquids may be unloaded separately but concurrently to minimize delivery time.
To maintain a constant and high discharge pressure, on-board compression is used to unload the modules in a combined and sequential order.
To increase capacity and efficiency, refrigeration and insulation may be used.
In order to maintain continuous production, several shuttle tankers may be required.
A truck-mounted CNG/CNGL transportation system will also have the pressure vessels mounted vertically. This will allow for the lowest center of gravity possible in the presence of any liquids. Similarly, the truck-mounted system has an upper gas manifold and a lower liquids manifold, connecting the banks of pressure vessels. At the end of each manifold exists an isolation and control valve.
The truck-trailer is designed and constructed to combine the trailer body and the support frame for the pressure vessels. This allows for a high strength trailer body, using minimal materials, translating into a light-weight transportation system.
A modular CNG/CNGL transportation system is similar in that the composite pressure vessels are vertically oriented and are integrally connected by an upper gas manifold and a lower liquids manifold. The pressure vessels are nested and the support frame is designed and constructed integral with the modular container frame.
The transportation of liquefied ethane by the above described means may include conventional refrigeration and insulation to insure that a liquid state is maintained.
Having thus described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments.